The Process and Equipment
Because most engineering thermoplastic parts are fabricated by injection molding, it is important for the designer to understand the molding process, its capabilities and its limitations. The basic process is very simple. Thermoplastic resins such as Delrin® acetal resins, Crastin® and Rynite® thermoplastic polyester resins, or Zytel® nylon resins, supplied in pellet form, are dried when necessary, melted, injected into a mold under pressure and allowed to cool. The mold is then opened, the parts removed, the mold closed and the cycle is repeated.
The Molding Machine
Melting the plastic and injecting it into the mold are the functions of the plastifying and injection system. The rate of injection and the pressure achieved in the mold are controlled by the machine hydraulic system. Injection pressures range from 35-140 MPa. Melt temperatures used vary from a low of about 215° C for Delrin® acetal resins to a high of about 300° C for some of the glass reinforced Zytel® nylon and Rynite® polyester resins. Processing conditions, techniques and materials of construction for molding DuPont Engineering Thermoplastic Resins can be found in the Molding Guides available for Delrin® acetal resins, Minlon® engineering thermoplastic resins, Crastin® and Rynite® thermoplastic polyester resins and Zytel® nylon resins.
Mold design is critical to the quality and economics of the injection molded part. Part appearance, strength, toughness, size, shape, and cost are all dependent on the quality of the mold. Key considerations for Engineering Thermoplastics are:
Proper design for strength to withstand the high pressure involved.
Correct materials of construction, especially when reinforced resins are used.
Properly designed flow paths to convey the resin to the correct location in the part.
Proper venting of air ahead of the resin entering the mold.
Carefully designed heat transfer to control the cooling and solidification of the moldings.
Easy and uniform ejection of the molded parts.
When designing the part, consideration should be given to the effect of gate location and thickness variations upon flow, shrinkage, warpage, cooling, venting, etc. Your DuPont representative will be glad to assist with processing information or mold design suggestions. The overall molding cycle can be as short as two seconds or as long as several minutes, with one part to several dozen ejected each time the mold opens. The cycle time can be limited by the heat transfer capabilities of the mold, except when machine dry cycle or plastifying capabilities are limiting.